Resistance to cytotoxic agents is a major limitation of their clinical use for the treatment of human cancers. Tumors become resistant to chemotherapy when a subset of cells undergoes molecular changes leading to over-expression of drug transport proteins, alterations in drug-target interactions or reduced cell ability to commit apoptosis. However, such changes may not be sufficient to explain why both resistant and nonresistant cells survive the drug action in tumors that ultimately become drug resistant. We hypothesize that, in such tumors, a cytoprotective relationship exists between drug resistant and sensitive cells. This hypothesis was addressed by study of putative cytoprotective signaling between doxorubicin resistant neuroblastoma cells and their drug sensitive counterparts. The drug resistant cells displayed enhanced expression of the drug efflux pump P-glycoprotein as a predicted mechanism of acquired drug resistance. Interestingly, these cells were found to secrete in their culture medium, factors able to protect drug sensitive cells from doxorubicin-induced apoptotic cell death. This was associated with inhibition of the apoptotic molecule caspase-3 and activation of molecules believed to mediate anti-apoptosis such as Stat3 and Akt. A survival molecule, midkine, identified by cDNA array in drug resistant cells, was believed to play a key role in this cytoprotective phenomenon. Midkine-enriched fractions obtained by affinity chromatography exert a significant cytoprotective effect against doxorubicin. These findings suggest that inter-cellular cytoprotective signals, originating from cells with acquired drug resistance to protect neighboring drug sensitive cells, may contribute to development of drug resistance. This investigation will focus on the following: 1) characterization of the cytoprotective action of midkine in drug sensitive neuroblastoma cells, 2) analysis of the mechanism(s) underlying midkine action, and 3) determination of midkine's effect on other cellular constituents of drug resistant tumors such as fibroblasts and endothelial cells. If established, this type of intercellular cross-talk will constitute a novel component of the multifactorial drug resistance phenomenon, and may facilitate the design of adequate treatment strategies.